A new IT application called cloud computing is drawing attention as a big flow of IT trend of recent years. STaaS (Storage as a service) as an example of the IT application which provides a storage service via the Internet is proliferating. The STaaS is a service of providing a logical storage capacity at a low price, and the STaaS generally provides an external interface for using the storage service in a Web interface format, such as REST and SOAP. Although problems, such as security and compliance, remain in the STaaS, the cost reduction is prioritized, and the use of the STaaS by individuals and companies is expected to increase in the future.
In general, the online storage service features an enormous scalability, but there is an upper limit to the file size that can be stored. For example, in the case of Amazon (registered trademark) S3, although there is no limit to the number of file objects that can be stored in the bucket, the maximum size of the file object is 5 GB, and a single file in a size greater than 5 GB cannot be stored (constraint condition 1). As for file reading, a partial reading process can be executed. However, as for writing, partial writing cannot be performed, and only the replacement of the file stored in the bucket is possible (constraint condition 2). For example, to rewrite a 1 MB block in a 5 GB file, all the 5 GB file needs to be downloaded to rewrite the 1 MB block, and the entire 5 GB file needs to be uploaded at the end. There is a problem that the communication cost increases. Therefore, there is a problem that data communication of an unnecessary part is generated in a Random READ/WRITE process, and the communication cost increases. The data is accessed via a WAN, and the communication speed is significantly slower than the data communication speed to a local disk or the like. There is a problem that the overhead of file listing or READ/WRITE increases in a synchronous communication.
To use the online storage service as a backup external storage of a file server or an external storage for capacity expansion, a mechanism is necessary in which the file server looks like a transparently single file server from the client, and the online storage service is connected from the file server to copy or move data. For example, Patent Literature 1 discloses an existing technique of transparently showing the file server as a single server to the client and concealing the data copy and movement. Patent Literature 2 discloses an existing technique related to the data copy and movement.
In Patent Literature 1, an intermediate apparatus is arranged between a first server including a primary storage, a second server including a secondary storage, and the client. When there is a file access to the primary storage from the client, the access request is changed to a corresponding file on the secondary storage if the access target file is a stub file (file for linking the destination). In this way, it looks as if the stub file is an entity, and the data movement from the primary storage to the secondary storage is concealed from the client.
In Patent Literature 2, updates of high-speed disk volumes are monitored to create update bit maps, OR of each update bit map is calculated to detect segments that are not updated for a predetermined time, and the data is moved from the high-speed disk volumes to low-speed disk volumes (movement between storage tires) based on the detected segment information.